admin.php

<?php
/**
 * Admin
 */
class WC_Category_Locker_Admin
{
    /**
     * Constructor
     */
    public function __construct()
    {
        add_action('product_cat_add_form_fields', [$this, 'add_category_fields'], 25);
        add_action('product_cat_edit_form_fields', [$this, 'edit_category_fields'], 25);
        add_action('created_term', [$this, 'save_category_fields'], 10, 3);
        add_action('edit_term', [$this, 'save_category_fields'], 10, 3);
    }

    /**
     * Add category fields
     *
     * @since 1.0
     * @return void
     */
    public function add_category_fields()
    {
        ?>
        <div class="form-field">
            <label id="wcl_cat_password_protected">
                <input type="checkbox" name="wcl_cat_password_protected" value="1" />
                <?php _e('Password Protected', WCL_PLUGIN_DOMAIN); ?>
            </label>
            <div id="wcl_cat_password" style="display:none; float: left;">
                <label>
                    <?php _e('Password:', WCL_PLUGIN_DOMAIN); ?>
                    <input type="text" name="wcl_cat_password" value="" required="required" />
                </label>
            </div>
            <script>
                jQuery('#wcl_cat_password_protected').on('click', function() {
                    var $checked = jQuery('input[name="wcl_cat_password_protected"]:checkbox:checked').length > 0;
                    if($checked) {
                    jQuery('#wcl_cat_password').find('input').prop('disabled', false);
                    jQuery('#wcl_cat_password').slideDown();
                    } else {
                    jQuery('#wcl_cat_password').find('input').prop('disabled', true);
                    jQuery('#wcl_cat_password').slideUp();
                    }
                });
            </script>
            <div class="clear"></div>
        </div>
    <?php
    }

    /**
     * Edit category fields
     *
     * @param object $term
     * @since 1.0
     * @return void
     */
    public function edit_category_fields($term)
    {
        $wcl_cat_password_protected = absint(get_woocommerce_term_meta($term->term_id, 'wcl_cat_password_protected', true));
        $wcl_cat_password = get_woocommerce_term_meta($term->term_id, 'wcl_cat_password', true); ?>
        <tr class="form-field">
            <th scope="row" valign="top">
                Password Protection
            </th>
            <td>
                <label id="wcl_cat_password_protected">
                    <input type="checkbox" name="wcl_cat_password_protected" value="1"
                    <?php
                    if ($wcl_cat_password_protected) {
                        echo 'checked="checked"';
                    } ?> />
                    <?php _e('Password Protected', WCL_PLUGIN_DOMAIN); ?>
                </label>
                <div class="clear"></div>
                <div id="wcl_cat_password" style="<?php
                    if (!$wcl_cat_password_protected) {
                        echo 'display:none;';
                    } ?> float: left;">
                  <label>
                      <?php _e('Password:', WCL_PLUGIN_DOMAIN); ?>
                      <input type="text" name="wcl_cat_password" value="<?php echo $wcl_cat_password; ?>" <?php
                      if (!$wcl_cat_password_protected) {
                          echo 'disabled="disabled"';
                      } ?> required="required" />
                  </label>
                </div>
                <script>
                    jQuery('#wcl_cat_password_protected').on('click', function() {
                        var $checked = jQuery('input[name="wcl_cat_password_protected"]:checkbox:checked').length > 0;
                        if($checked) {
                          jQuery('#wcl_cat_password').find('input').prop('disabled', false);
                          jQuery('#wcl_cat_password').slideDown();
                        } else {
                          jQuery('#wcl_cat_password').find('input').prop('disabled', true);
                          jQuery('#wcl_cat_password').slideUp();
                        }
                    });
                </script>
                <div class="clear"></div>
            </td>
        </tr>
      <?php
    }

    /**
     * Save category fields
     *
     * @param integer $term_id
     * @param string $tt_id
     * @param string $taxonomy
     * @since 1.0
     * @return void
     */
    public function save_category_fields($term_id, $tt_id = '', $taxonomy = '')
    {
        if (isset($_POST['wcl_cat_password_protected']) && 'product_cat' === $taxonomy) {
            update_woocommerce_term_meta($term_id, 'wcl_cat_password_protected', absint($_POST['wcl_cat_password_protected']));
        } elseif ('product_cat' === $taxonomy) {
            update_woocommerce_term_meta($term_id, 'wcl_cat_password_protected', 0);
        }

        if (isset($_POST['wcl_cat_password']) && 'product_cat' === $taxonomy) {
            update_woocommerce_term_meta($term_id, 'wcl_cat_password', esc_attr($_POST['wcl_cat_password']));
        }
    }
}

// init
$WC_Category_Locker_Admin = new WC_Category_Locker_Admin();